It is reported that (2R)-2-propyloctanoic acid is useful for preventing and/or treating agent for neurodegenerative diseases such as brain infarction and Alzheimer's disease, since it has an ability to improve functions of astrocyte (see the specification of European Patent No. 0632008). Also, it is known that (2R)-2-propyloctanoic acid is useful for preventing and/or treating agent for Parkinson's disease or Parkinson's syndrome (see the specification of European Patent No. 1174131). Moreover, it is known that (2R)-2-propyloctanoic acid is useful for preventing and/or treating agent for cranial nerve diseases described hereinbefore, since it has an ability to decrease intracellular content of S100β to improve functions of astrocyte (see The Journal of Cerebral Blood Flow & Metabolism, 22, 723-734 (2002)). Furthermore, it is known that (2S)-2-propynylheptanoic acid derivatives functionate as a neurotrophic factor (see the specification of U.S. Pat. No. 5,672,746).